locale - build failure with recommended icu configuration

[hvemha@…]

Configuring icu as recommended here:
​http://source.icu-project.org/repos/icu/icu/trunk/readme.html#RecBuild
More precisely, build fails if icu::UnicodeString contructors are declared explicit. The attach patch fixes build errors.

All fixes in formatter.cpp constructs UnicodeStrings from invariant strings (invariant characters defined here: ​http://icu-project.org/apiref/icu4c/platform_8h.html#a7fb0b0fede299f9d74973b15e79d3085). Previously all these constructors would involve an extra overhead as they were depending on the conversion framework.

The change in time_zone.cpp still depends on the conversion framework (behavior not changed). I do not think it is possible to guarantee that the provided string only includes invariant characters. In this case, the resulting string is undefined.